To determine whether the non-viral carcinogens that do induce mammary tumors in mice do so by modification of MuMTV gene expression, or whether they act by an MuMTV independent, alternative pathway. In order to be able to be able to answer this question, we intend to study biochemical, virological and immunological parameters reflecting MuMTV gene expression at short intervals, in individual mice, which are part of large groups, each of which is subjected to a different oncogenic stimulus, or to a combination of such stimuli.